Chlorine from the mantle: Magmatic halides in the Udachnaya-East kimberlite, Siberia

Abstract

The Devonian Udachnaya-East pipe (Siberia) presents a rare example of exceptionally fresh kimberlite rocks, containing a rich assemblage of alkali carbonate, chloride and sulphate minerals in its groundmass. Accordingly, bulk groundmass compositions have unusually high concentrations of sodium and chlorine (up to 6 wt.% each), while water contents are very low. High Na2O and low H2O are inconsistent with currently accepted model kimberlite magma compositions; and high Na and Cl contents in the Udachnaya-East pipe have been ignored by the geoscience community, because of possibility of contamination from sedimentary crustal rocks. New textural evidence suggests that the chloride and alkali carbonate minerals in this kimberlite are co-magmatic with perovskite. Radiogenic isotope ratios for the perovskite (87Sr/86Sr ~0.7031, Nd +5, Hf +5.3, obtained by solution-mode and laser-ablation methods) are very primitive and provide no support for a crustal contaminant. The new perovskite data confirm previously published Sr-Nd-Pb isotopic evidence for the chlorides and alkali carbonates themselves, but are considered more reliable because of possible open system behaviour and analytical problems with unstable halide and carbonate minerals. Similar isotope ratios (87Sr/86Sr=0.70292, Nd=+5, Hf=+6.5) are recorded in a clinopyroxene macrocryst, which contains chlorides and alkali carbonates within melt inclusions, implying a similarly primitive isotopic signature for the host kimberlite. We infer that assimilation of evaporitic sediments is unlikely and that abundant chlorine and sodium in the kimberlite originated in the upper mantle. Our results and published experimental data on mantle chloride melts and presence of a chloride component in diamond-hosted fluid inclusions suggest a previously unrecognised role for mantle-derived halide melts in the genesis, composition and rheological properties of kimberlite primary melts.